1. Technical Field
The present disclosure relates to a separator, and a nonaqueous electrolytic secondary battery, such as a lithium ion battery, including the same.
2. Related Art
A nonaqueous electrolytic secondary battery such as a lithium ion battery includes an electrode assembly and a case for housing the electrode assembly. The electrode assembly is a stack of a positive electrode, a negative electrode, and a separator interposed therebetween. The electrode assembly is housed in the case in a state that the electrode assembly is wound. An electrolyte is held between the positive electrode and the negative electrode.
One surface or both surfaces of the separator interposed between the positive electrode and the negative electrode of the nonaqueous electrolytic secondary battery is/are coated with a mixture including an inorganic particle and a binder. Application of this mixture provides the separator with an inorganic layer. The porosity of this inorganic layer per unit area is higher than the porosity of a polyolefin layer of the separator. Therefore, the inorganic layer is capable of holding a larger amount of electrolytic solution. The inorganic layer thus contributes to improvement of durability of the separator. The separator has a function to hold the electrolyte. The separator further has a function to prevent short-circuiting due to contact between the positive electrode and the negative electrode.
When a nonaqueous electrolytic secondary battery is left under high temperature or subjected to repeated charge-discharge cycles, the electrolyte existing between the positive electrode and the negative electrode moves to a dead space between the case and the electrode assembly. This results in decrease in the absolute amount of the electrolyte between the electrodes. It therefore leads to a problem of deterioration in durability of the secondary battery.
In a nonaqueous electrolytic secondary battery disclosed in JP-A-2005-353452, the pore ratio of a portion closer to a case is lower than the pore ratio of a portion far from the case. In other words, in the winding type, the pore ratio of a portion of an electrode group that is closer to an end thereof in the short-side direction is lower than that of an inner portion.
An object of the structure of this secondary battery is to improve the function of the separator to hold the electrolyte. However, for achieving this structure, the pore ratio of the separator itself needs to be changed for every portion. This makes it difficult to assemble the electrode assembly and fabricate the separator.
JP-A-2008-140551 discloses a stacked nonaqueous electrolytic secondary battery. In this secondary battery, the periphery of the separator includes a low-pore-ratio part having the lower pore ratio than the other portions.
The structure disclosed in JP-A-2008-140551 has been made in consideration that short-circuiting between a positive electrode active material layer and a negative electrode active material layer intensively occurs in the periphery. That is, falloff of a conductive particle from the periphery of the active material layer is suppressed by making the pore ratio of the periphery of the separator lower than that of the other portions. This can prevent the short-circuiting between the positive electrode active material layer and the negative electrode active material layer. In other words, in this structure, it is difficult to improve the function of the separator to hold the electrolyte. The pore ratio of the separator is hereinafter referred to as porosity.
In the technique proposed in JP-A-H10-50287, a microporous film having heat resistance is stacked on a surface of a separator. This suppresses contraction of the separator during high-temperature treatment. Moreover, in the technique disclosed in JP-A-2008-243660, a contraction suppression layer for a separator is not formed at a portion where a positive electrode current collector exposure part and a negative electrode current collector exposure part face each other. For this reason, the current collector exposure parts are brought into contact with each other due to contraction of the current collector exposure parts during high-temperature treatment. This leads to early decrease in potential of the positive electrode. Therefore, reliability of the battery is improved. The function of the separator to hold the electrolyte is therefore hardly improved even with this technique.